Western Pacific Tropical Cyclone Research Articles

Synthetically impacts of the topography and typhoon periphery on the atmospheric boundary layer structure and special regional pollution pattern of O3 in North China Plain

Westerly warm advection from the Loess Plateau to the over layer of North China Plain (NCP) suppresses the mixed layer during the daytime in summer, having already be confirmed as an important formation mechanism of regional O3 pollution all over NCP. However, when NCP was simultaneously affected by the westerly warm advection from the Loess Plateau and the typhoon periphery airflow from the Western Pacific tropical cyclone system, a rare pollution pattern of surface O3 was detected, characterized by a very low ozone pool in Beijing (LOP-BJ) contrast to much higher O3 in surrounding areas. To explore the causes and deep formation mechanism to this special pollution phenomenon, regional pollution characteristics of O3, characteristics of the atmospheric boundary layer (ABL) structure and vertical airflows affected by different weather systems were analyzed based on the analysis of observations and three dimensional simulations. It was found that the synthetic impacts of the topography and the typhoon peripheral flow promoted the stabilization of the ABL and introduced a nearly isolated air mass in terms of both thermodynamic and dynamic structures within ABL in Beijing region. In the stable and isolated ABL air masses, the significant lower ozone near the ground was primarily attributed to the strong loss by NO titration consumption from local excessive emissions and extremely weak transport contributions from surrounding areas. The conclusion was also confirmed by the integrated process rate (IPR) analysis results of contributions from chemistry tendency (CHEM) and total transport tendency (TRAN) to surface ozone. Our work insight the understanding of the formation mechanism of regional ozone pollution and improve the forecasting capabilities under comprehensive weather conditions over NCP.

Mechanisms of tropical cyclone response under climate change in the community earth system model

Climate change induces a myriad of effects which influences the global tropical cyclone (TC) genesis frequency. Here we explore how North Atlantic and Western Pacific TCs are affected under climate change using a present-day and a future (1% pCO2 scenario) ensemble of high resolution simulations. We find that the number of TCs decreases (-45%) in the North Atlantic but increases (+15%) in the Western Pacific. Part of these opposing variations are linked to differences in the ocean’s meridional overturning circulation, which gives rise to a different sea surface temperature response and air-sea fluxes between the two basins. The results show the important role of oceanic climate change on TC response.

Raindrop Size Distribution Characteristics of the Western Pacific Tropical Cyclones Measured in the Palau Islands

Due to the severe threat of tropical cyclones to human life, recent years have witnessed an increase in the investigations on raindrop size distributions of tropical cyclones to improve their quantitative precipitation estimation algorithms and modeling simulations. So far, the raindrop size distributions of tropical cyclones using disdrometer measurements have been conducted at coastal and inland stations, but such studies are still missing for oceanic locations. To the authors’ knowledge, the current study examines—for the first time—the raindrop size distributions of fourteen tropical cyclones observed (during 2003–2007) at an oceanic station, Aimeliik, located in the Palau islands in the Western Pacific. The raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands showed unlike characteristics between stratiform and convective clusters, with a larger mass-weighted mean diameter and smaller normalized intercept parameter in the convective type. The contribution of the drop diameters to the total number concentration showed a gradual decrease with the increase in drop diameter size. Raindrop size distributions of Western Pacific tropical cyclones measured in the Palau islands differed slightly from Taiwan and Japan. The helpfulness of empirical relations in raindrop size distribution in rainfall estimation algorithms of ground-based (Z–R, μ–Λ, Dm–R, and Nw–R) and remote-sensing (σm–Dm, μo–Dm, Dm–Zku, and Dm–Zka) radars are evaluated. Furthermore, the present study also related the rainfall kinetic energy of fourteen tropical cyclones with rainfall rate and mass-weighted mean diameter (KEtime–R, KEmm–R, and KEmm–Dm). The raindrop size distribution empirical relations appraised in this study offer a chance to: (1) enhance the rain retrieval algorithms of ground-based, remote sensing radars; and (2) improve rainfall kinetic energy estimations using disdrometers and GPM DPR in rainfall erosivity studies.

Three Western Pacific Typhoons Strengthened Fire Weather in the Recent Northwest U.S. Conflagration

AbstractStrong winds that accentuated a fire outbreak in the western United States in early September of 2020 resulted from an atmospheric wave train that spanned the Pacific Ocean. Days before the atmospheric waves developed in the United States, three western Pacific tropical cyclones (typhoons) underwent an extratropical transition over Korea within an unprecedentedly short span of 12 days. Using a climate diagnostic approach and historical forecast data, it was found that the amplitude of the atmospheric waves accompanying the western U.S. fire weather would not have been so profound if not for the influence of these typhoons. Together, the recurving typhoons provided a significant source of wave activity flux directed toward North America – amplifying the ridge over the U.S. west coast while deepening the trough in central Canada. This anomalous circulation produced the severe frontal system that caused extreme winds in western Oregon, Washington and California – rapidly spreading fire.

North Western Pacific Tropical Cyclone Ensemble Forecast Project

ABSTRACT The WMO/TCP and WWRP launched the North Western Pacific Tropical Cyclone Ensemble Forecast Project (NWP-TCEFP) in 2009 to explore the utility of ensemble forecasts, including multi-model ensemble forecasts of TCs, and to promote such products for operational TC forecasting. Operational global medium-range ensembles, which have been exchanged in real-time in a CXML format under the initiative of the WMO GIFS-TIGGE Working Group, have been used to create ensemble products of TC tracks that were used by the Typhoon Committee Members and forecasters participating in the SWFDP in Southeast Asia through a password-protected website developed and maintained by the JMA. In some cases many or all of the ensembles have simultaneously predicted small or large ensemble spreads in TC tracks. The implication is that multi-model ensemble products provide forecasters with additional information on forecast certainty or uncertainty and thus increase the level of confidence in the forecasts. Another important outcome of the project was the responses to surveys conducted by the WMO/TCP and WWRP and also by the WMO GIFS-TIGGE Working Group. The responses confirmed that ESCAP/WMO Typhoon Committee Members have routinely accessed the website and have recognized the usefulness of the ensemble products available on the website for operational TC forecasting.

Interaction of western-Pacific tropical cyclones with El Niño diversity

Interaction of western-Pacific tropical cyclones with El Niño diversity

A dynamical‐statistical forecast model for the annual frequency of western Pacific tropical cyclones based on the NCEP Climate Forecast System version 2

A dynamical‐statistical forecast model for the annual tropical cyclones over the western North Pacific is developed based on the empirical relationship between the actual annual number of tropical cyclones (ANTCs) and the dynamical predictions of large‐scale variables by the Climate Forecast System version 2 of the National Centers for Environmental Prediction (NCEP). On interannual time scales, the ANTCs are significantly and negatively correlated with the July–October tropical North Atlantic sea surface temperature, tropical western Pacific vertical zonal wind shear (WPVZWS), and subtropical Pacific geopotential height at 500 hPa (HGT500). They are also positively correlated with the zonal wind at 850 hPa over the tropical Pacific Ocean. Skillful forecasts of the above four potential predictors are made with the 24‐member ensemble predictions by the NCEP model. The two‐predictor model with the HGT500 and the WPVZWS shows the most skillful hindcasts at 0–2 month leads assessed by the leave‐one‐out cross validation for the ANTCs over the 31 year record between 1982 and 2012. The corresponding correlation coefficients and the root‐mean‐square errors (RMSEs) between the observed and hindcast ANTCs are in the ranges from 0.73 to 0.79 and from 3.11 to 2.75, respectively. Observed ANTCs during El Niño–Southern Oscillation events are generally well captured with RMSEs ranging from 3.12 to 3.04 during El Niño years and from 3.62 to 2.44 during La Niña years. The forecast skill of the model for the past 10 years (2003–2012) is competitive with the current forecast schemes. The forecast model initialized in March, May, and June 2013 suggests an inactive season for 2013, with about 22 tropical cyclones.

Changes in Western Pacific Tropical Cyclones Associated with the El Niño–Southern Oscillation Cycle

Abstract This study examines the interannual variability of three groups of tropical cyclones (TCs)—super typhoons (STYs), typhoons (TYs), and tropical storms and tropical depressions (TSTDs)—and their relationship with the El Niño–Southern Oscillation (ENSO). Both wavelet analysis and correlation studies of upper-ocean heat content reveal significant differences for the three types of TCs. In particular, an increase (decrease) in the frequency of STYs is usually associated with the mature phase of El Niño (La Niña) events, while the converse is true for TSTDs. In contrast, the frequency of TYs increases (decreases) during the transition period from La Niña to El Niño (El Niño to La Niña) events. The results suggest that the timing with which ENSO impacts STYs, TYs, and TSTDs varies and that their corresponding changes in frequency closely follow the evolution of the ENSO cycle. Empirical orthogonal function analysis is also conducted to investigate the impact of different environmental factors influenced by ENSO on TCs. The vertical wind shear and moist static energy associated with ENSO are identified as the dominant factors that control the frequency of STYs. In comparison, the frequency of TYs is found to be closely related to the relative vorticity and vertical wind shear associated with both the transition phase of ENSO and with other types of climate variability.

Quasi-decadal spectral peaks of tropical western Pacific SSTs as a precursor for tropical cyclone threat

[1] A recent study identified significant spectral peaks in sea surface temperatures (SSTs) at quasi-decadal frequency bands, which were unique to the tropical western Pacific (TWP). Using the multitaper method for spectral and coherence analysis, this study finds that the TWP SSTs at these quasi-decadal time scales are coherent with western Pacific tropical cyclone threat [as measured by the power dissipation index (PDI)], but that the PDI lags TWP SSTs by about two years. Thus, the quasi-decadal peaks in TWP SSTs may be a precursor for enhanced tropical cyclone threat two years later. Composite analyses are shown to illustrate how areal tropical cyclone frequencies and relevant atmospheric fields evolve following the quasi-decadal peaks in TWP SSTs. The fields during peak PDI years strongly resemble those observed during El Nino events.

The Interannual Stability of Cumulative Frequency Distributions for Convective System Size and Intensity

Abstract The characteristics of convective system populations in West Africa and the western Pacific tropical cyclone basin were analyzed to investigate whether interannual variability in convective activity in tropical continental and oceanic environments is driven by variations in the number of events during the wet season or by favoring large and/or intense convective systems. Convective systems were defined from Tropical Rainfall Measuring Mission (TRMM) data as a cluster of pixels with an 85-GHz polarization-corrected brightness temperature below 255 K and with an area of at least 64 km2. The study database consisted of convective systems in West Africa from May to September 1998–2007, and in the western Pacific from May to November 1998–2007. Annual cumulative frequency distributions for system minimum brightness temperature and system area were constructed for both regions. For both regions, there were no statistically significant differences between the annual curves for system minimum brightness temperature. There were two groups of system area curves, split by the TRMM altitude boost in 2001. Within each set, there was no statistically significant interannual variability. Subsetting the database revealed some sensitivity in distribution shape to the size of the sampling area, the length of the sample period, and the climate zone. From a regional perspective, the stability of the cumulative frequency distributions implied that the probability that a convective system would attain a particular size or intensity does not change interannually. Variability in the number of convective events appeared to be more important in determining whether a year is either wetter or drier than normal.

Foreshowing of the Western Pacific tropical cyclone track to PM10 air pollution episode in the Beijing area

By utilizing the air quality monitoring data and the NCEP reanalysis data, the relationship between the PM10 air pollution episode and synoptic situation is analyzed in the Beijing area. It is interesting to find that PM10 air pollution episode in and around the Beijing area is correlated with the Western Pacific tropical cyclone track to some extent, namely when a tropical cyclone lands southward to the Changjiang River, PM10 air pollution episode is not easy to take place generally in the Beijing area; but when a tropical cyclone moves northward and finally lands at the Korea Peninsula or the Japanese mainland, and under this condition the Beijing area is generally controlled by weak high or weak low for several days, PM10 air pollution episode often takes place in one day or several days. Above findings indicate that predicting the Western Pacific tropical cyclone track can foretell whether or not PM10 air pollution episode takes place in the Beijing area, which can offer a technique for urban air quality prediction and air pollution source management in the Beijing area.

Transition from an eastern Pacific upper‐level mixed Rossby‐gravity wave to a western Pacific tropical cyclone

The ancestor of unseasonal Typhoon Nanmadol (2004) over the western North Pacific is traced back to the Eastern Pacific (near 120°W) as an upper tropospheric, counter‐clockwise rotating mixed Rossby‐gravity (MRG) wave. The temporal and spatial evolution from an equatorial trapped MRG‐wave‐type disturbance to an off‐equatorial tropical depression is documented by utilizing NOGAPS (Navy's Operational Global Atmospheric Prediction System) analysis data. It is found that the MRG wave moved away from the equator after a dramatic reduction in its zonal dimension and downward development and amplification in the lower troposphere. These dramatic changes in MRG wave properties are attributed to the modulation of the easterly vertical shear, low‐level westerly‐easterly confluence in the large‐scale background flows, and convective coupling, as well as underlying high sea surface temperature (SST). The boundary layer convergence associated with asymmetric pressure field of the MRG wave is likely responsible for the transition from an equatorial MRG to an off‐equatorial tropical disturbance. This case study addresses the possibility that upper tropospheric MRG waves over the far eastern Pacific may provide “seeds” for tropical cyclogenesis over the western North Pacific.

The ocean response to operational westerly wind bursts during the 1991–1992 El Niño

Numerical simulations of the remotely forced ocean response to westerly wind bursts prior to and during the 1991–1992 El Niño are examined; the models are forced by wind stress from the U.S. Navy's atmospheric global operational analysis/forecast system. The study focuses on (1) the relative response of the first and second internal modes to a single episode of westerly bursts; (2) the role of westerly bursts in producing the eastern Pacific sea level variations from October 1990 to February 1992; and (3) the relative importance of the remotely forced sea level responses generated by central and western Pacific wind anomalies. The simulations use the Naval Research Laboratory global multilayer formulation; the suite of experiments includes hydrodynamic simulations that use both one‐ and three‐active‐layer reduced gravity models as well as an experiment that also includes thermodynamic effects. The models are executed from January 1, 1990, to March 1, 1992, a period that includes 10 significant westerly wind bursts or burst clusters and the 1991–1992 El Niño event. The numerical experiments reveal an ability to accurately simulate the eastern Pacific sea level variations during this period. In response to a single burst the three‐layer hydrodynamic simulation reveals that the second internal mode Kelvin wave yields a sea level change at the eastern boundary that is approximately one third that of the first mode and a surface velocity signature that is equivalent to the first mode. During the onset of the El Niño event the inclusion of higher modes also produces a more realistic representation of the observed eastern boundary sea level signal. Furthermore, by comparing the model response to particular wind bursts with the observed sea level at Baltra, Galapagos, a value of 2.5–2.6 m/s is suggested as the most appropriate mean speed for the first internal mode Kelvin wave during the onset phase. The models reveal the following scenario for the onset of the 1991–1992 El Niño. The eastern boundary sea level exhibited three, distinct pulselike events superimposed on a general rise; the observed sea level pulses occurred in October–November, December–January, and February. The numerical simulations indicate that the pulses in December–January and February were the result of three powerful, westerly wind bursts in the western Pacific preceded by westerly anomalies in the central Pacific. The Kelvin wave pulse generated in the western Pacific by the wind burst in late December to early January was reinforced by a strong central Pacific westerly wind anomaly in mid‐January. These westerly wind events were associated with the development of western Pacific tropical cyclones near the equator in one or both hemispheres.

International Experiments to Study Tropical Cyclones in the Western North Pacific

Four separate (but coincident in time) field experiments to study tropical cyclones in the western North Pacific area will be carried out during August/September 1990 by the United States, the Economic and Social Commission for Asia and the Pacific/World Meteorological Organization (ESCAP/WMO) Typhoon Committee, the USSR, and by Taiwanese scientists. The objective of the U.S. experiment is to improve basic understanding of tropical cyclone motion. The focus of the ESCAP/WMO Typhoon Committee experiment is to improve operational track prediction. A multi-ship and aircraft expedition is planned by the USSR to understand the effects of the ocean inhomogeneities on tropical cyclones, and the response of the ocean to typhoon passage. Researchers in Taiwan will organize special observations of typhoons approaching Taiwan to understand the wind and precipitation distributions. The combined observations from these experiments should result in a comprehensive dataset for the study of western Pacific tropical cyclones.

Long term variations in western tropical Pacific cyclogenesis associated with the Southern Oscillation

THE ‘Southern Oscillation’, as statistically defined by Sir Gilbert Walker, is the barometrically recorded exchange of air mass in tropical latitudes around the complete circumference of the globe1,2. Indices of Southern Oscillation based on the surface pressure differences between the Indonesian equatorial low (measured at Darwin, Australia) and the South Pacific subtropical high (considered the Southern Oscillation's eastern core and measured at Easter Island) have been used by Quinn3–5, who relates fluctuations in these indices to anomalous equatorial rainfall and occurrences of ‘El Nino’, (the accumulation of anomalously warm water in the eastern tropical Pacific). In this paper, we use 12-month running means of monthly mean surface pressures over three areas across the northern tropical Pacific as an index of the Southern Oscillation index, and we find that pressure convergences in this index are associated with years of relatively low numbers of western Pacific tropical cyclones.